Angiopoietin-2 specific TIE2 receptor

ABSTRACT

In one aspect, provided herein is a polypeptide comprising a modified angiopoietin receptor or fragment thereof, wherein the polypeptide binds preferentially to angiopoietin-2 compared to angiopoietin-1. Nucleic acid sequences encoding the polypeptide, as well as pharmaceutical uses of the polypeptide in treating diseases such as cancer and inflammation are also provided.

FIELD

The present invention relates to polypeptides useful for treatingdiseases in humans and animals. In particular, the invention relates topolypeptide inhibitors of angiopoietin-2 and their use in treatingdiseases such as cancer.

BACKGROUND

Angiopoietin-2 (Ang2) is a 70 kDa secreted ligand whose increasedexpression has been implicated in a range of diseases, including cancer,sepsis and adult respiratory distress syndrome (1, 2). The primaryreceptor for Ang2 is the transmembrane tyrosine kinase Tie2 (3) that isexpressed mainly on vascular endothelial cells and myeloid cells (1, 4).Ang2 plays an important role in vascular remodeling during developmentbut in adult tissues Ang2 concentrations are usually low. An increase inAng2 levels in disease allows the molecule to compete for binding to acommon interface on Tie2 with the related agonist Ang1 (3). Ang1 is aprotective protein constitutively produced by perivascular cells whichmaintains blood vessel function and quiescence by suppressinginflammation, vessel leakage and endothelial apoptosis (1, 5).Antagonism of Ang1 by Ang2 blocks the pro-quiescent effects of Ang1 andcontributes to Ang2-induced vessel remodelling, inflammation, leakageand oedema. In addition to its actions on endothelial Tie2, Ang2 has anumber of other effects relevant to disease. For example, the ligand hasrecently been shown to bind and activate endothelial integrins topromote sprouting angiogenesis (6), and Ang2 acts on tumour infiltratingTie2-expressing monocytes to promote tumourigenesis (7, 8).

Because of its involvement in multiple disease processes there have beenconsiderable efforts to develop inhibitors of Ang2, including antibodiesand aptamers (9-11). Results from studies with these and relatedmolecules have been encouraging, with reports of Ang2 inhibitorspromoting tumor regression and suppressing of metastatic disease incancer, and decreasing leukocyte infiltration and vascular remodeling inairway inflammation (7, 10, 12, 13).

A complementary approach to the use of antibodies for blockingpathological levels of ligands is the cytokine or ligand trap (14).These molecules are formed from receptor ectodomain fragments, usuallyadministered as soluble fusion proteins, which sequester the targetligand. Examples of ligand traps in clinical use include Etanercept, asoluble form of tumour necrosis factor-α receptor and Aflibercept, achimeric fusion protein of fragments of vascular endothelial growthfactor receptor-1 and -2 (15). There are significant advantages toligand traps. Usually they are smaller and have better tissuepenetration than antibodies, they already recognize the biologicallyactive part of the target and generally do not require protection fromthe immune system. A ligand trap specific for Ang2 would be anattractive therapeutic. However the natural receptor for Ang2, Tie2,binds to the protective ligand Ang1 equally well or even better than itdoes to Ang2 (3, 16, 17).

One of the most effective strategies for engineering new proteinfunctionality is directed protein evolution (18, 19). This processessentially recapitulates the selection and accumulation of desirablemutations that occurs in natural evolution over millions of years, butover a period of weeks in the laboratory. Directed evolution involvesrepeated rounds of library construction, usually in vitro, expression ofthe mutant forms of the target protein and selection. Unfortunately thisiterative approach to in vitro generation and searching of sequencespace is frequently both difficult and labour intensive. B cell linesthat constitutively diversify their immunoglobulin variable (IgV)regions by somatic hypermutation (SHM) (20) allow for coupling ofdiversification and selection of novel antibody specificities. Thegenetic variation within the Ig genes, introduced by the action ofactivation induced deaminase (AID) is coupled to the selectableexpression of surface Ig on individual cells (21). More recently suchcell lines have been used to evolve variants of exogenously expressedgreen fluorescent protein (22, 23). However, in theory this strategy hasenormous potential for directed evolution of a wide range of proteins ifthe desired phenotype can be selected for in B lines.

There is thus still a need for an improved inhibitor of Ang2. Inparticular, there is a need for a polypeptide angiopoietin inhibitorwhich is capable of discriminating between Ang2 and Ang1.

SUMMARY

In one aspect the present invention provides a polypeptide comprising amodified angiopoietin receptor or fragment thereof, wherein thepolypeptide binds preferentially to angiopoietin-2 compared toangiopoietin-1.

In one embodiment, the angiopoietin receptor is Tie2. Preferably thepolypeptide comprises a modified Tie2 ectodomain.

In one embodiment, the polypeptide comprises a variant of human Tie2comprising 1 to 30 amino acid variations with respect to SEQ ID NO:1 orSEQ ID NO:2 or a fragment thereof.

In another embodiment, the polypeptide comprises a variant of SEQ IDNO:2 or residues 23-210 of SEQ ID NO:1, the variant comprising 1 to 30amino acid substitutions, deletions or insertions compared to SEQ IDNO:2 or residues 23-210 of SEQ ID NO:1.

In another embodiment, the polypeptide has at least 90% sequenceidentity to at least 50 amino acid residues of SEQ ID NO:1 or SEQ IDNO:2.

The polypeptide preferably comprises one or more mutations with respectto SEQ ID NO:1 or SEQ ID NO:2 or a fragment thereof selected from:F161G, F161I, ΔR167, ΔH168, V154L, P171A, E169D, V170I and T226S.

In a preferred embodiment, the polypeptide comprises the mutation F161I.In another preferred embodiment, the polypeptide comprises the mutationF161G. In another preferred embodiment, the polypeptide comprises themutation ΔR167/ΔH168. In a particularly preferred embodiment, thepolypeptide comprises the mutations F161I, ΔR167 and ΔH168. In anotherparticularly preferred embodiment, the polypeptide comprises themutations F161G, ΔR167 and ΔH168.

In one embodiment, the polypeptide has at least 90% sequence identity toat least 50 amino acid residues of SEQ ID NO:3, e.g. the polypeptide maycomprise at least 50 amino acid residues of SEQ ID NO:3.

In some embodiments, fragment as described above are at least 50 aminoacid residues in length.

In one embodiment, the polypeptide binds to Ang2 and Ang1 with anaffinity ratio of at least 10:1. For instance, the polypeptide may bindto Ang2 with a Kd of less than 10 nM, and/or the polypeptide may bind toAng1 with a Kd of greater than 1 μM.

In a further aspect, the invention provides a nucleic acid encoding apolypeptide as described above.

In one embodiment, the nucleic acid comprises a variant of SEQ ID NO:4or SEQ ID NO:5 or a portion thereof comprising one or more nucleotidesubstitutions, deletions or insertions as shown in FIG. 9 or FIG. 10A.

In a further aspect, the invention provides an expression vectorcomprising a nucleic acid as described above.

In a further aspect, the invention provides a host cell comprising anexpression vector as described above.

In a further aspect, the invention provides a pharmaceutical compositioncomprising a polypeptide or nucleic acid as described above and apharmaceutically acceptable carrier, diluent or excipient.

In a further aspect, the invention provides a polypeptide, nucleic acidor pharmaceutical composition as described above, for use in theprevention or treatment of an angiopoietin-2-mediated disease orcondition.

In a further aspect, the invention provides use of a polypeptide,nucleic acid or pharmaceutical composition as described above, for thepreparation of a medicament for preventing or treating anangiopoietin-2-mediated disease or condition.

In a further aspect, the invention provides a method for preventing ortreating an angiopoietin-2-mediated disease or condition in a subject inneed thereof, comprising administering to the subject a pharmaceuticallyeffective amount of a polypeptide, nucleic acid or pharmaceuticalcomposition as described above.

In one embodiment, the disease or condition is cancer, inflammation,sepsis, angiogenesis, oedema, retinopathy, age-related maculardegeneration or hypertension.

Embodiments of the present invention provide a variant form of a Tie2ectodomain which preferentially binds Ang2 and which can be used toblock the damaging effects of this ligand without suppressing theprotective effects of Ang1. This was achieved by combining SHM-drivengene diversification with surface display in a B cell line to evolve aform of Tie2 ectodomain with preferential binding to Ang2.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1. Directed evolution of receptor ectodomain. (A) Strategy fordirected evolution in hypermutating B cells. (B) Alignment of thereceptor binding P-domains of human Ang1 (SEQ ID NO: 7) and Ang2 (SEQ IDNO: 8). (C) Schematic representation of the surface expression constructincorporating residues 1-442 of Tie2 and used for directed evolution.(D) Anti-FLAG immunofluorescent staining of DT40 cells transfected withsurface expression construct. (E) Flow cytometry of DT40 cellsexpressing Tie2 ectodomain. Untransfected (left plot) and transfected(right plot) cells were allowed to bind His₆-tagged 1 nM Ang1 or Ang2 orno ligand for 30 min before staining with anti-His and fluorescentsecondary antibody.

FIG. 2. Evolution of ligand-specific Tie2 ectodomain. (A) FACS plots ofDT40 cells following incubation with 1 nM Ang1 and staining withanti-Ang1 and fluorescent secondary antibody together with fluorescentanti-FLAG (Expression). Polygons indicate the gates used to select thecells on sorts 1 (upper plot, left), 2 (upper plot, center) and 4 (upperplot, right) sorts. Cells from sort 4 were then incubated with 1 nM Ang1and 1 nM biotinylated Ang2 and binding detected withanti-Ang1/fluorescent secondary antibody and fluorescently labelledstreptavidin. Cells were selected for highest Ang2 binding. Polygonsindicate gates used to select cells on sorts 5 (lower plot, left) and 6(lower plot, center) sorts. After 8 sorts (lower plot, right) cells wereselected for sequencing as indicated by the polygon. (B) Comparison ofDT40 cells expressing wild-type receptor with the evolved (R3)population of cells for binding of 1 nM Ang1 and Ang2. Grey plots showfluorescence following staining in the absence of ligand for eachpopulation of cells.

FIG. 3. Three amino acid changes switch the binding specificity of Tie2.(A) The primary sequence shown is residues 1 to 442 of human Tie2,defined herein as SEQ ID NO: 2. Twenty random sequences from R3 cellswere determined and all demonstrated F161I substitution and R167, H168deletion. The sequence shown in this Figure comprising the F161Isubstitution and R167/H168 deletion is defined herein as SEQ ID NO: 3.(B) The F161I substitution is positioned on a beta sheet and thedeletion on a turn at the receptor:ligand interface. Top=Ang2;Bottom=Tie2. Modelled on PDB accession 2GY7 (26).

FIG. 4. Evolved ectodomain binds specifically to Ang2. (A) Secretedwild-type and evolved ectodomains were purified following expression inHEK293 cells and immobilized on SPR sensors. Analysis of Ang1 and Ang2binding to wild-type or evolved ectodomains is shown. (B) Secretedwild-type ectodomain and ectodomains with either F161I substitution orthe double R167, H168 deletion were expressed, purified and analysed forbinding to immobilized Ang1 or Ang2 by ELISA. Data are shown as meansand standard deviations from a single experiment with triplicatedeterminations performed at least three times.

FIG. 5. Evolved ectodomain blocks the effects of Ang2 on endothelialcells. (A) The antagonistic effects of Ang2 on Ang1-activation of Aktphosphorylation were tested in the endothelial cell line EA.hy926. Cellswere activated with 50 ng/ml Ang1 in the absence and presence of 200ng/ml Ang2 and 25 μg/ml wild-type (Wt) or R3 ectodomain for 30 minbefore cell lysis, gel electrophoresis and immunoblotting withantibodies recognizing Akt phosphorylated on S473 (pAkt) or total AKT(tAkt) as indicated. (B) The agonist activity of 1 μg/ml Ang2 onactivation of Akt phosphorylation was tested in the absence and presenceof 25 μg/ml R3, for comparison the effects on 50 ng/ml Ang1 are alsoshown. In order to see the low level of pAkt induced by Ang2 blots wereoverexposed resulting in the appearance of additional non-specificbands, pAkt is indicated with an arrow. (C) Migration of endothelialcells in response to high concentrations of Ang2 (1 μg/ml) was inhibitedby R3 ectodomain whereas this mutant ectodomain did not affect migrationin response to Ang1 (50 ng/ml). Data are shown as means and SEM forthree independent experiments.

FIG. 6. Plasmid map of the Tie2-Hypermut2 surface expression plasmid.RSV promoter and downstream Tie2 surface display sequence is shown alongwith Ig homology regions, SV40 polyA sequences, and beta-actin promoter.The approximate position of the Not1 restriction site used for plasmidlinearization is also shown.

FIG. 7. Targeted integration of Tie2-Hypermut2 into DT40 Ig locus. (A)Schematic representation of unrearranged and rearranged Ig locus andTie2-Hypermut2 with regions of homology (pink) and integrated construct.The positions of primers P4 and P5 are indicated. PCR of DT40 genomicDNA from transfectants with P4/P5 amplify a 493 bp segment, confirmed in(B) for three representative clones, if integration has not occurred inthe unrearranged locus. PCR amplification of genomic DNA fromtransfected DT40 with primers GW1/GW2 amplify a 1189 bp segment whenTie2-Hypermut2 inegrates into rearranged locus, shown for three clonesin (C). Control amplifications (Cont) without DNA and amplificationsfrom untransfected DT40 genomic DNA (Unt) are also shown, ns indicates anon-specific amplification product.

FIG. 8. Anti-FLAG immunoblot of DT40 cells. Cell lysates were preparedfrom untransfected (Unt) DT40 and three transfected clones andimmunoblotted for the FLAG epitope tag.

FIGS. 9A-9B. Mutations in non expressing DT40 population. Genomic DNAwas prepared from non-expressing DT40, selected by FACS followingstaining with anti-FLAG and Ang1 binding, and used for amplification ofDNA encoding Tie2 ectodomain. Thirty randomly selected colonies weresequenced following transformation into E Coli. The primary nucleic acidsequence shown (designated herein SEQ ID NO: 4) encodes the human Tie2ectodomain, i.e. residues 1-442 of SEQ ID NO: 1 (which is designatedherein as SEQ ID NO: 2). For mutated nucleotides, the substitutednucleotide shown is above, dash indicates a deletion. Some mutations didnot affect expression but these were accompanied by a deletion thatablated expression.

FIG. 10. Mutations in R3 Ang2-specific binding population. Genomic DNAwas prepared from the R3 population shown in FIG. 5 and used foramplification of DNA encoding Tie2 ectodomain. Twenty randomly selectedcolonies were sequenced following transformation into E Coli. (A) Theprimary nucleic acid sequence shown (designated herein SEQ ID NO: 5)comprises residues 401-750 of SEQ ID NO: 4. For mutated nucleotides, thesubstituted nucleotide is shown above, dash indicates a deletion. Thenucleotide changes found in all twenty sequences are underlined. (B) Theprimary amino acid sequence shown (designated herein SEQ ID NO:6)comprises residues 101-300 of SEQ ID NO:1. Amino acid changes resultingfrom mutations are shown, changes found in all twenty sequences areunderlined.

FIG. 11. Purified soluble ectodomain-Fc fusion proteins. Coomassiestained gel of wild type (Wt) and R3 ectodomain-Fc fusion proteinsfollowing expression in Hek 293 cells and purification on nickelcolumns. The positions of mass markers are indicated in kDa.

FIGS. 12A-12C. Amino acid sequence of human Tie 2. The full length aminoacid sequence of human Tie2 (SEQ ID NO:1), as described in databaseaccession no. Q02763, is shown.

FIG. 13. Evolved ectodomain suppresses localized oedema in vivo.Quantitative analysis of local oedema formation in mice injected withcontrol carrier (black), LPS, LPS with R3 ectodomain or LPS withinactive ΔR167,H168 ectodomain. Data from individual mouse hocks takentwo hours post-injection are presented as mean subcutis thickness(distance between tibial periost and epidermis), minimum and maximumvalues +/−SD and compared to the matched controls for a minimum of ninedata points (*P<0.005; **P<0.0001, Students ‘t’ test). The experimentwas performed at two independent times (mice 99-101 and 67-69, 72, 73)with the same stock of LPS.

FIG. 14. Evolved ectodomain suppresses localized oedema in vivo.Quantitative analysis of local oedema formation in mice injected withcontrol carrier, LPS or LPS with R3 ectodomain, as indicated. Data fromindividual mouse hocks taken one hour post-injection are presented asmean subcutis thickness and SD for four data points per mouse. Data areshown for each matched pair of mice.

FIGS. 15A-B. SPR analysis of R3 I161G mutant binding showing the mutantdoes not bind Ang1 but shows increased Ang2 binding compared with R3.

FIG. 16. Elisa binding of R3 and R3 I161 G mutant to immobilised Ang2shows R3 I161 G mutant has improved Ang2 binding.

DETAILED DESCRIPTION

In one aspect the present invention relates to a polypeptide comprisinga modified angiopoietin receptor or fragment thereof, wherein thepolypeptide binds preferentially to angiopoietin-2 compared toangiopoietin-1.

Angiopoietin Receptors

By “angiopoietin receptor” it is meant an agent which binds selectivelyor specifically to angiopoietin. Preferably the angiopoietin receptor isTie2 (Tyrosine kinase with Ig and EGF homology domains-2), which mayalso be known as: Tyrosine-protein kinase receptor TIE-2; Angiopoietin-1receptor; Endothelial tyrosine kinase; Tunica interna endothelial cellkinase; Tyrosine-protein kinase receptor TEK; p140 TEK; and CD antigen202b. Tie2 is classified as a receptor tyrosine kinase in classEC=2.7.10.1 according to the IUBMB Enzyme Nomenclature. The amino acidsequence of human Tie2 may be found under UniProtKB/Swiss-Prot databaseaccession number Q02763, and is shown in SEQ ID NO:1 (FIG. 12).

Modified Angiopoietin Receptors and Fragments Thereof

The polypeptide described herein comprises a modified angiopoietinreceptor or fragment thereof. By “modified” it is meant that thepolypeptide sequence comprises one or more differences (e.g. amino acidsubstitutions, deletions or insertions) with respect to a wild typeangiopoietin receptor, e.g. compared to human Tie2 (Q02763, as shown inSEQ ID No:1 and FIG. 12). The polypeptide may thus be a variant, mutantor other modified form of an angiopoietin receptor, preferably of humanTie2.

Preferably the polypeptide comprises at least two or at least threeamino acid changes with respect to the wild type angiopoietin receptor.In particular embodiments, the polypeptide may comprise 1 to 30, 1 to20, 1 to 10, 1 to 5, 2 to 30, 2 to 20, 2 to 10 or 2 to 5 amino aciddifferences compared to a corresponding sequence in the wild typereceptor or a fragment thereof, e.g. compared to human Tie2 (SEQ IDNO:1) or a fragment thereof.

Amino acid changes may include substitutions, deletions or insertions.Substitutional variants are those that have at least one amino acidresidue in a native sequence removed and a different amino acid insertedin its place at the same position. The substitutions may be single,where only one amino acid in the molecule has been substituted, or theymay be multiple, where two or more amino acids have been substituted inthe same molecule.

Insertional variants are those with one or more amino acids insertedimmediately adjacent to an amino acid at a particular position in anative sequence. Immediately adjacent to an amino acid means connectedto either the alpha-carboxy or alpha-amino functional group of the aminoacid.

Deletional variants are those with one or more amino acid residues in anative sequence removed. For example, deletional variants may have one,two or more amino acid residues deleted in a particular region of themolecule. Deletional mutations are represented herein by the symbol A.

By “fragment” it is meant a portion of the full length sequence of anangiopoietin receptor, typically which is capable of foldingindependently and/or which retains one or more structural or biologicalproperties of the full length sequence. Thus fragments as describedherein are capable of preferentially binding to Ang2 compared to Ang1.Preferred fragments are typically 10 to 1000, 20 to 800, 30 to 500, 30to 800, 30 to 500, 50 to 500, 50 to 300, or 100 to 200 amino acidresidues in length.

In some embodiments, the fragment comprises substantially all, or atleast a portion of, the extracellular domain (ectodomain) of theangiopoietin receptor. The term “extracellular domain” or “ectodomain”refers to the amino acid sequences in an angiopoietin receptor that arenormally exposed on the outer surface of the cell membrane and which aretypically involved in binding to Ang2. Extracellular and ligand bindingdomains in angiopoietin receptors may be determined by methods known inthe art, including X-ray studies, mutational analyses, and antibodybinding studies. The mutational approaches include the techniques ofrandom saturation mutagenesis coupled with selection of escape mutants,and insertional mutagenesis. Another strategy suitable for identifyingligand-binding domains in receptors is known as alanine (Ala)-scanningmutagenesis. See e.g. Cunningham, et al., Science 244, 1081-1985 (1989).This method involves the identification of regions that contain chargedamino acid side chains. The charged residues in each region identified(i.e. Arg, Asp, His, Lys, and Glu) are replaced (one region per mutantmolecule) with Ala and the ligand binding of the obtained receptors istested, to assess the importance of the particular region in ligandbinding. A further method for the localization of ligand binding domainsis through the use of neutralizing antibodies. Usually a combination ofthese and similar methods is used for localizing the domains which areextracellular and are involved in binding to Ang2.

In one embodiment, the polypeptide comprises an amino acid sequencewhich is homologous to at least residues 1-442 or residues 23-210 ofhuman Tie2. Residues 1-442 of human Tie2 are shown in FIG. 3A and aredefined herein as SEQ ID NO:2. For instance, the polypeptide maycomprise a sequence which is a variant or homologue of residues 1-442 orresidues 23-210 of SEQ ID NO:1, e.g. comprising 1 to 30, 1 to 10 or 1 to5 amino acid substitutions, deletions or additions compared to residues1-442 or residues 23-210 of SEQ ID NO:1. In further embodiments, thepolypeptide may comprise a variant or homologue of at least residues100-210, 150-210 or 150-170 of SEQ ID NO:1.

Preferably, the modified angiopoietin receptor or fragment thereof showsat least 70%, at least 80%, at least 90%, at least 95%, at least 97%, atleast 98% or at least 99% homology or sequence identity to a portion ofthe wild type angiopoietin receptor, e.g. over at least 30, at least 50,at least 100, at least 200, at least 300 or at least 500 amino acidresidues or over the full length of the sequence. The term “homology”can be equated with “sequence identity”. For instance, the polypeptidemay have any of the above degrees of sequence identity to SEQ ID NO:1,SEQ ID NO:2 or a fragment thereof, e.g. over at least 30, 100 or 300amino acid residues of SEQ ID NO:1 or SEQ ID NO:2 or to residues 1-442or residues 23-210 of SEQ ID NO:1.

Sequence identity comparisons can be conducted by eye, or more usually,with the aid of readily available sequence comparison programs. Thesecommercially available computer programs use complex comparisonalgorithms to align two or more sequences that best reflect theevolutionary events that might have led to the difference(s) between thetwo or more sequences. Therefore, these algorithms operate with ascoring system rewarding alignment of identical or similar amino acidsand penalising the insertion of gaps, gap extensions and alignment ofnon-similar amino acids. The scoring system of the comparison algorithmsinclude:

-   -   i) assignment of a penalty score each time a gap is inserted        (gap penalty score),    -   ii) assignment of a penalty score each time an existing gap is        extended with an extra position (extension penalty score),    -   iii) assignment of high scores upon alignment of identical amino        acids, and    -   iv) assignment of variable scores upon alignment of        non-identical amino acids.

Most alignment programs allow the gap penalties to be modified. However,it is preferred to use the default values when using such software forsequence comparisons.

The scores given for alignment of non-identical amino acids are assignedaccording to a scoring matrix also called a substitution matrix. Thescores provided in such substitution matrices are reflecting the factthat the likelihood of one amino acid being substituted with anotherduring evolution varies and depends on the physical/chemical nature ofthe amino acid to be substituted. For example, the likelihood of a polaramino acid being substituted with another polar amino acid is highercompared to being substituted with a hydrophobic amino acid. Therefore,the scoring matrix will assign the highest score for identical aminoacids, lower score for non-identical but similar amino acids and evenlower score for non-identical non-similar amino acids. The mostfrequently used scoring matrices are the PAM matrices (Dayhoff et al.(1978), Jones et al. (1992)), the BLOSUM matrices (Henikoff and Henikoff(1992)) and the Gonnet matrix (Gonnet et al. (1992)).

Suitable computer programs for carrying out such an alignment include,but are not limited to, Vector NTI (Invitrogen Corp.) and the ClustalV,ClustalW and ClustalW2 programs (Higgins D G & Sharp P M (1988), Higginset al. (1992), Thompson et al. (1994), Larkin et al. (2007). A selectionof different alignment tools are available from the ExPASy Proteomicsserver at www.expasy.org. Another example of software that can performsequence alignment is BLAST (Basic Local Alignment Search Tool), whichis available from the webpage of National Center for BiotechnologyInformation which can currently be found athttp<colon-slash-slash>www.ncbi.nlm.nih.gov/ and which was firstlydescribed in Altschul et al. (1990) J. Mol. Biol. 215; 403-410.

Once the software has produced an alignment, it is possible to calculate% similarity and % sequence identity. The software typically does thisas part of the sequence comparison and generates a numerical result.

In one embodiment, it is preferred to use the ClustalW software forperforming sequence alignments. Preferably, alignment with ClustalW isperformed with the following parameters for pairwise alignment:

Substitution matrix: Gonnet 250 Gap open penalty: 20 Gap extensionpenalty: 0.2 Gap end penalty: None

ClustalW2 is for example made available on the internet by the EuropeanBioinformatics Institute at the EMBL-EBI webpage www<dot>ebi.ac.uk undertools—sequence analysis—ClustalW2. Currently, the exact address of theClustalW2 tool is www<dot>ebi.ac.uk/Tools/clustalw2.

In another embodiment, it is preferred to use the program Align X inVector NTI (Invitrogen) for performing sequence alignments. In oneembodiment, Exp10 has been may be used with default settings:

Gap opening penalty: 10

Gap extension penalty: 0.05

Gap separation penalty range: 8

Score matrix: blosum62mt2

Preferred Mutations

In some embodiments, the polypeptide comprises one or more mutationscompared to the wild type Tie2 ectodomain sequence as described below inthe Examples. In one embodiment, the polypeptide comprises one or moremutations (e.g. substitutions, deletions or insertions) at residues 150to 230 of the human Tie2 sequence (SEQ ID NO:1) or a fragment thereof(e.g. SEQ ID NO:2). Preferably the polypeptide comprises one or moremutations within the region 150 to 180, more preferably 160 to 175, mostpreferably 160 to 170 of SEQ ID NO:1 or 2.

In one embodiment, the polypeptide comprises a mutation at one or moreof the following positions in the human Tie2 sequence (SEQ ID NO:1) or afragment thereof (e.g. SEQ ID NO:2): 154, 161, 167, 168, 169, 170, 171and 226. Preferably the polypeptide comprises a mutation at one, two orthree of positions 161, 167 and 168 of SEQ ID NO:1 or 2.

Preferably the polypeptide comprises one or more of the followingmutations with respect to the human Tie2 sequence (SEQ ID NO:1) or afragment thereof (e.g. SEQ ID NO:2): F161G, F161I, ΔR167, ΔH168, V154L,P171A, E169D, V170I and T226S.

In one embodiment, the polypeptide comprises the mutation F161I. Inanother embodiment, the polypeptide comprises the mutation ΔR167/ΔH168.In one embodiment, the polypeptide comprises at least the followingcombination of mutations: F161I, ΔR167 and ΔH168, e.g. with respect toSEQ ID NO:1 or SEQ ID NO:2.

In one embodiment, the polypeptide comprises the mutation F161G. Inanother embodiment, the polypeptide comprises the mutation ΔR167/ΔH168.In one embodiment, the polypeptide comprises at least the followingcombination of mutations: F161G, ΔR167 and ΔH168, e.g. with respect toSEQ ID NO:1 or SEQ ID NO:2.

In a particularly preferred embodiment, the polypeptide comprises atleast 30, at least 50, as least 100, at least 200, at least 300 aminoacid residues, or the full length of SEQ ID NO:3, i.e. the sequence ofSEQ ID NO:2 modified by the mutations F161I, ΔR167 and ΔH168 (see FIG.3A). Variants and homologues of SEQ ID NO:3 are also contemplated, e.g.comprising 1 to 30, 1 to 10 or 1 to 5 amino acid substitutions,deletions or additions compared to SEQ ID NO:3, provided that themutations F161I, ΔR167 and ΔH168 are present. In further embodiments,the polypeptide may comprise a variant or homologue of at least residues100-210, 150-210 or 150-170 of SEQ ID NO:3. Sequences showing at least70%, at least 80%, at least 90%, at least 95%, at least 97%, at least98% or at least 99% homology or sequence identity to at least 30, atleast 50, at least 100, at least 200, at least 300 or at least 500 aminoacid residues of, or over the full length of, SEQ ID NO:3 are alsodescribed, provided that the mutations F161I, ΔR167 and ΔH168 areretained.

In another embodiment, the polypeptide comprises a variant of SEQ IDNO:3 comprising the mutation I161G (with respect to SEQ ID NO:3), or avariant or homologue thereof as described in the preceding paragraph.The mutation I161G with respect to SEQ ID NO:3 corresponds to themutation F161G with respect to SEQ ID NO:2. Thus in some embodiments thepolypeptide comprises at least 70%, 90% or 95% sequence identity to atleast 30, at least 100 or over the full length of SEQ ID NO:3, providedthat the mutations F161G, ΔR167 and ΔH168 with respect to SEQ ID NO:2are present.

Further Mutations

Further modified angiopoietin receptors comprising alternative mutationsmay be constructed using methods analogous to those described herein,with particular reference to the Examples below. For instance, methodsfor evolving proteins with specificity for a selected target using invitro somatic hypermutation in cell lines are described in e.g.WO00/22111, WO02/100998 and WO03/095636.

Preferential Binding to Ang2

The polypeptides of the present invention bind preferentially to Ang2compared to Ang1. In other words, the polypeptides are typicallyselective for Ang2 over Ang1, e.g. the polypeptides bind with higheraffinity to Ang2 than to Ang1 under the same conditions. Bindingaffinity may be measured using standard techniques known in the art,e.g. surface plasmon resonance, ELISA and so on (for instance asdescribed below in the Examples), and may be quantified in terms ofeither association (K_(a)) or dissociation (K_(d)) constants.

In a preferred embodiment, the polypeptide binds to Ang2 and Ang1 withan affinity ratio of at least 2:1 (e.g. K_(a) (Ang2)/K_(a) (Ang1)≥2). Infurther embodiments, the polypeptide may have an affinity ratio forAng2/Ang1 of at least 5:1, at least 10:1, at least 100:1, at least1000:1 or at least 10,000:1. For instance, the polypeptide may bind toAng2 with a K_(d) of less than 100 μM, preferably less than 1 μM, morepreferably less than 100 nM, most preferably less than 10 nM. Thepolypeptide may bind to Ang1 with a K_(d) of greater than 10 nM,preferably greater than 100 nM, more preferably greater than 1 μM, mostpreferably greater than 100 μM. In one embodiment the polypeptide doesnot bind to Ang1 (e.g. the polypeptide shows negligible or substantiallyno binding to Ang1 under standard assay conditions).

Nucleic Acids, Expression Vectors and Host Cells

Nucleic acid sequences encoding the above-described polypeptides arealso provided herein. Suitable nucleic acid sequences can be preparedusing methods known in the art based on the published sequences ofangiopoietin receptors such as human Tie2. A nucleic acid sequenceencoding residues 1-442 of human Tie2 (i.e. the ectodomain) is shown inFIG. 9 (SEQ ID NO:4). Residues 401 to 750 of SEQ ID NO:4 are shown inFIG. 10A (SEQ ID NO:5).

Variant nucleic acid sequences comprising mutations which encodepolypeptides according to the present invention are also shown in FIGS.9 and 10A. Typically such variant sequences show at least 50%, at least70%, at least 80%, at least 90%, at least 95%, at least 97% or at least99% sequence identity to SEQ ID NO:4 or SEQ ID NO:5 or a portionthereof, e.g. over at least 50, 100, 200, 500 or 1000 nucleotideresidues or over the full length of either sequence, provided that thesequence comprises at least one of the mutations shown in FIG. 9A or 10.Sequence identity may be determined as described above in relation topolypeptide sequences.

Variant nucleic acid sequences encoding modified angiopoietin receptorsare readily prepared by methods known in the art, such as by sitedirected mutagenesis of the DNA encoding the native receptor. Suchsequences can be cloned into suitable vectors for expression of thedesired recombinant polypeptide in host cells. The term “recombinant”refers to proteins that are produced by recombinant DNA expression in ahost cell. The host cell may be prokaryotic (for example, a bacterialcell such as E. coli) or eukaryotic (for example, a yeast or a mammaliancell). For example, a nucleic acid encoding the polypeptide may beplaced into an expression vector, which is then transfected into hostcells such as simian COS cells or Chinese Hamster ovary (CHO) cells. Therecombinant host cells are grown in suitable culture medium, and thedesired fragment or amino acid sequence variant expressed in the hostcells is recovered from the recombinant cell culture by chromatographicor other purification methods.

Conjugates and Fusion Proteins

In some embodiments the polypeptides described herein may be conjugatedto further moieties which augment their biological activity. Forexample, the polypeptides may be fused with heterologous polypeptides,such as viral sequences, with cellular receptors, with cytokines such asTNF, interferons, or interleukins, with polypeptides having procoagulantactivity, with cytotoxins, and with other biologically orimmunologically active polypeptides. For instance, in one embodiment itmay be desirable to kill cells which express Ang2, and this may beachieved by conjugating a cytotoxin (e.g. diptheria, ricin orPseudomonas toxin, or a chemotherapeutic agent) to the polypeptidedescribed above. Such fusions are readily made either by recombinantcell culture methods (e.g. where the polypeptide is fused to a furtherpolypeptide moiety) or by covalently crosslinking the cytotoxic moietyto an amino acid residue side chain or C-terminal carboxyl of thepolypeptide, using methods such as disulfide exchange or linkage througha thioester bond (e.g. using iminothiolate andmethyl-4-mercaptobutyrimadate).

Diagnostic Uses

The polypeptides described herein may be used in various methods fordetecting Ang2, either in vitro or in vivo. For diagnostic applications,the polypeptides may be labeled with a detectable moiety. The detectablemoiety can be any one which is capable of producing, either directly orindirectly, a detectable signal. For example, the detectable moiety maybe a radioisotope, such as ³H, ¹⁴C, ³²P, ³⁶S, or ¹²⁵I, a fluorescent orchemiluminescent compound, such as fluorescein isothiocyanate,rhodamine, or luciferin; radioactive isotopic labels, such as, e.g.,¹²⁶I, ³²P, ¹⁴C, or ³H, or an enzyme, such as alkaline phosphatase,beta-galactosidase or horseradish peroxidase.

Any method known in the art for separately conjugating the polypeptideto the detectable moiety may be employed, including those methodsdescribed by Hunter, et al., Nature 144:945 (1962); David, et al.,Biochemistry 13:1014 (1974); Pain, et al., J. Immunol. Meth. 40:219(1981); and Nygren, J. Histochem. and Cytochem. 30:407 (1982).

The polypeptides described herein may be employed in any assay format,such as competitive binding assays, direct and indirect sandwich assays,and precipitation assays for detecting Ang2.

Competitive binding assays rely on the ability of a labeled standard(which may be labelled Ang2) to compete with the test sample analyte(e.g. human Ang2) for binding with a limited amount of the polypeptidesdescribed herein. The amount of Ang2 in the test sample is inverselyproportional to the amount of standard that becomes bound to thepolypeptide. To facilitate determining the amount of standard thatbecomes bound, the polypeptide may be insolubilized before or after thecompetition, so that the standard and analyte that are bound to thepolypeptide may conveniently be separated from the standard and analytewhich remain unbound.

Sandwich assays involve the use of two polypeptides, each capable ofbinding to a different portion of the protein to be detected. In asandwich assay, the test sample analyte is bound by a first polypeptidewhich is immobilized on a solid support, and thereafter a secondpolypeptide binds to the analyte, thus forming an insoluble three partcomplex. See e.g. David & Greene, U.S. Pat. No. 4,376,110. The secondpolypeptide may itself be labeled with a detectable moiety (directsandwich assays) or may be measured using an antibody that is labeledwith a detectable moiety (indirect sandwich assay). For example, onetype of sandwich assay is an ELISA assay, in which case the detectablemoiety is an enzyme.

The polypeptides described herein may also be useful for in vivoimaging, wherein a polypeptide labeled with a detectable moiety isadministered to a patient, preferably into the bloodstream, and thepresence and location of the labeled polypeptide in the patient isassayed. This imaging technique may be useful, for example, in thestaging and treatment of neoplasms. The polypeptide may be labeled withany moiety that is detectable in a mammal, whether by nuclear magneticresonance, radiology, or other detection means known in the art.

Pharmaceutical Formulations

The polypeptides described herein may be formulated into variouscompositions for pharmaceutical use. Such dosage forms encompasspharmaceutically acceptable carriers that are inherently nontoxic andnontherapeutic. Examples of such carriers include ion exchangers,alumina, aluminum stearate, lecithin, serum proteins, such as humanserum albumin, buffer substances such as phosphates, glycine, sorbicacid, potassium sorbate, partial glyceride mixtures of saturatedvegetable fatty acids, water, salts, or electrolytes such as protaminesulfate, disodium hydrogen phosphate, potassium hydrogen phosphate,sodium chloride, zinc salts, colloidal silica, magnesium trisilicate,polyvinyl pyrrolidone, cellulose-based substances, and polyethyleneglycol. Carriers for topical or gel-based forms of the polypeptideinclude polysaccharides such as sodium carboxymethylcellulose ormethylcellulose, polyvinylpyrrolidone, polyacrylates,polyoxyethylene-polyoxypropylene-block polymers, polyethylene glycol,and wood wax alcohols. For all administrations, conventional depot formsare suitably used. Such forms include, for example, microcapsules,nano-capsules, liposomes, plasters, inhalation forms, nose sprays,sublingual tablets, and sustained-release preparations. The polypeptidewill typically be formulated in such vehicles at a concentration ofabout 0.1 mg/ml to 100 mg/mi.

Suitable examples of sustained release preparations includesemipermeable matrices of solid hydrophobic polymers containing thepolypeptide, which matrices are in the form of shaped articles, e.g.films, or microcapsules. Examples of sustained-release matrices includepolyesters, hydrogels (for example, poly(2-hydroxyethyl-methacrylate) asdescribed by Langer et al., J. Biomed. Mater. Res. 15:167 (1981) andLanger, Chem. Tech., 12: 98-105 (1982), or poly(vinylalcohol),polylactides (U.S. Pat. No. 3,773,919), copolymers of L-glutamic acidand gamma ethyl-L-glutamate (Sidman et al., Biopolymers, 22:547 (1983),non-degradable ethylene-vinyl acetate (Langer et al., supra), degradablelactic acid-glycolic acid copolymers such as the Lupron Depot™(injectable micropheres composed of lactic acid-glycolic acid copolymerand leuprolide acetate), and poly-D-(−)-3-hydroxybutyric acid. Whilepolymers such as ethylene-vinyl acetate and lactic acid-glycolic acidenable release of molecules for over 100 days, certain hydrogels releaseproteins for shorter time periods. When encapsulated polypeptides remainin the body for a long time, they may denature or aggregate as a resultof exposure to moisture at 37° C., resulting in a loss of biologicalactivity. Rational strategies can be devised for stabilization dependingon the mechanism involved. For example, if the aggregation mechanism isdiscovered to be intermolecular S—S bond formation throughthio-disulfide interchange, stabilization may be achieved by modifyingsulfhydryl residues, lyophilizing from acidic solutions, controllingmoisture content, using appropriate additives, and developing specificpolymer matrix compositions.

Sustained-release polypeptide compositions also include liposomallyentrapped forms. Liposomes containing the polypeptides may be preparedby methods known in the art, such as described in Epstein, et al., Proc.Natl. Acad. Sci. USA, 82:3688 (1985); Hwang, et al., Proc. Natl. Acad.Sci. USA, 77:4030 (1980); U.S. Pat. No. 4,485,045; U.S. Pat. No.4,544,545. Ordinarily the liposomes are the small (about 200-800Angstroms) unilamelar type in which the lipid content is greater thanabout 30 mol. % cholesterol, the selected proportion being adjusted forthe optimal HRG therapy. Liposomes with enhanced circulation time aredisclosed in U.S. Pat. No. 5,013,556.

Treatment of Angiopoietin-2 Related Diseases

For therapeutic applications, the polypeptides of the invention areadministered to a mammal, preferably a human, in a pharmaceuticallyacceptable dosage form, including those that may be administered to ahuman intravenously as a bolus or by continuous infusion over a periodof time, by intramuscular, intraperitoneal, intra-cerebrospinal,subcutaneous, intra-articular, intrasynovial, intrathecal, oral,topical, or inhalation routes. The polypeptides also are suitablyadministered by intratumoral, peritumoral, intralesional, orperilesional routes, to exert local as well as systemic therapeuticeffects. The intraperitoneal route may be particularly useful, forexample, in the treatment of ovarian tumors.

For the prevention or treatment of disease, the appropriate dosage ofpolypeptide will depend on the type of disease to be treated, theseverity and course of the disease, whether the polypeptides areadministered for preventive or therapeutic purposes, previous therapy,the patient's clinical history and response to the polypeptide, and thediscretion of the attending physician. The polypeptide is suitablyadministered to the patient at one time or over a series of treatments.

The polypeptides described herein are useful in the treatment of variousangiopoietin-2-related disorders, including neoplastic andnon-neoplastic diseases and disorders. The role of Ang2 in variousdiseases has been confirmed in numerous studies. For example, see thefollowing publications with respect to cancer (Oliner et al. 2004 CancerCell 6, 507-16; Mazzieri et al. 2011 Cancer Cell 19, 512-26; Thurston &Daly 2012, CSHLP Perspectives in Medicine); systemic inflammatorystates/sepsis (Thurston & Daly 2012, CSHLP Perspectives in Medicine);airway inflammation (Tabruyn et al 2010 Am J Pathol 177, 3233-3243);ocular neovascularisation: diabetic retinopathy, oxygen-inducedretinopathy in neonates, and age-related macular degeneration (Rennel etal. 2011 Microcirculation 18, 598-607); arteriovenous malformations(Hashimoto et al. 2001 Circ Res 89, 111-113); pulmonary hypertension(Dewachter et al2006 Am J Respir Crit Care Med 174, 1025-1033).

Neoplasms and related conditions that are amenable to treatment includebreast carcinomas, lung carcinomas, gastric carcinomas, esophagealcarcinomas, colorectal carcinomas, liver carcinomas, ovarian carcinomas,thecomas, arrhenoblastomas, cervical carcinomas, endometrial carcinoma,endometrial hyperplasia, endometriosis, fibrosarcomas, choriocarcinoma,head and neck cancer, nasopharyngeal carcinoma, laryngeal carcinomas,hepatoblastoma, Kaposi's sarcoma, melanoma, skin carcinomas, hemangioma,cavernous hemangioma, hemangioblastoma, pancreas carcinomas,retinoblastoma, astrocytoma, glioblastoma, Schwannoma,oligodendroglioma, medulloblastoma, neuroblastomas, rhabdomyosarcoma,osteogenic sarcoma, leiomyosarcomas, urinary tract carcinomas, thyroidcarcinomas, Wilm's tumor, renal cell carcinoma, prostate carcinoma,abnormal vascular proliferation associated with phakomatoses, edema(such as that associated with brain tumors), and Meigs' syndrome.

Non-neoplastic conditions that are amenable to treatment includeinflammation, including chronic inflammation and lung inflammation,sepsis, angiogenesis, oedema, diabetic and other retinopathies,age-related macular degeneration, hypertension rheumatoid arthritis,psoriasis, atherosclerosis, retrolental fibroplasia, neovascularglaucoma, thyroid hyperplasias (including Grave's disease), corneal andother tissue transplantation, nephrotic syndrome, preeclampsia, ascites,pericardial effusion (such as that associated with pericarditis), andpleural effusion.

Depending on the type and severity of the disease, about 1 μg/kg to 15mg/kg of polypeptide is an initial candidate dosage for administrationto the patient, whether, for example, by one or more separateadministrations, or by continuous infusion. A typical daily dosage mightrange from about 1 μg/kg to 100 mg/kg or more, depending on the factorsmentioned above. For repeated administrations over several days orlonger, depending on the condition, the treatment is repeated until adesired suppression of disease symptoms occurs. However, other dosageregimens may be useful. The progress of this therapy is easily monitoredby conventional techniques and assays, including, for example,radiographic tumor imaging.

According to another embodiment of the invention, the effectiveness ofthe polypeptide in preventing or treating disease may be improved byadministering the polypeptide serially or in combination with anotheragent that is effective for those purposes, such as tumor necrosisfactor (TNF), an antibody capable of inhibiting or neutralizing theangiogenic activity of vascular endothelial growth factor (VEGF), acidicor basic fibroblast growth factor (FGF) or hepatocyte growth factor(HGF), an antibody capable of inhibiting or neutralizing the coagulantactivities of tissue factor, protein C, or protein S (see Esmon, et al.,PCT Patent Publication No. WO 91/01753, published 21 Feb. 1991), or oneor more conventional therapeutic agents such as, for example, alkylatingagents, folic acid antagonists, anti-metabolites of nucleic acidmetabolism, antibiotics, pyrimidine analogs, 5-fluorouracil, purinenucleosides, amines, amino acids, triazol nucleosides, orcorticosteroids. Such other agents may be present in the compositionbeing administered or may be administered separately. Also, thepolypeptide is suitably administered serially or in combination withradiological treatments, whether involving irradiation or administrationof radioactive substances.

In one embodiment, vascularization of tumors is attacked in combinationtherapy. One or more polypeptides described herein are administered totumor-bearing patients at therapeutically effective doses as determinedfor example by observing necrosis of the tumor or its metastatic foci,if any. This therapy is continued until such time as no furtherbeneficial effect is observed or clinical examination shows no trace ofthe tumor or any metastatic foci. Then TNF is administered, alone or incombination with an auxiliary agent such as alpha-, beta-, orgamma-interferon, a VEGF antagonist, anti-HER2 antibody, heregulin,anti-heregulin antibody, D-factor, interleukin-1 (IL-1), interleukin-2(IL-2), granulocyte-macrophage colony stimulating factor (GM-CSF), oragents that promote microvascular coagulation in tumors, such asanti-protein C antibody, anti-protein S antibody, or C4b binding protein(see Esmon, et al., PCT Patent Publication No. WO 91/01753, published 21Feb. 1991), or heat or radiation.

Since the auxiliary agents will vary in their effectiveness it isdesirable to compare their impact on the tumor by matrix screening inconventional fashion. The administration of the polypeptide andauxiliary agent may be repeated until the desired clinical effect isachieved. In instances where solid tumors are found in the limbs or inother locations susceptible to isolation from the general circulation,the therapeutic agents described herein are administered to the isolatedtumor or organ. In other embodiments, a FGF or platelet-derived growthfactor (PDGF) antagonist, such as an anti-FGF or an anti-PDGFneutralizing antibody, is administered to the patient in conjunctionwith the polypeptide.

Other Uses

The polypeptides described herein are also useful as affinitypurification agents for Ang2. In this process, the polypeptides areimmobilized on a suitable support, such a Sephadex resin or filterpaper, using methods well known in the art. The immobilized antibodythen is contacted with a sample containing the Ang2 to be purified, andthereafter the support is washed with a suitable solvent that willremove substantially all the material in the sample except the Ang2,which is bound to the immobilized polypeptide. Finally, the support iswashed with another suitable solvent, such as glycine buffer, pH 5.0,that will release the Ang2 from the polypeptide.

The invention will now be further illustrated with reference to thefollowing non-limiting examples.

Example 1 Materials and Methods

Materials

cDNA encoding human Tie2 ectodomain (1-442), and platelet-derived growthfactor receptor 13 (residues 514-562 which includes the transmembranesequence) and with an amino terminal five alanine linker followed by theFLAG epitope, were generated by polymerase chain reaction. Theseamplification products were ligated into pcDNA3.1 and then transferredto the vector pHypermut2 (23). All constructs were verified bysequencing. Ang1, Ang2, biotinylated Ang2 and mouse Anti-Ang1 wereobtained from R & D Systems. Anti-FLAG conjugated to FITC andstreptavidin conjugated to phyoerythrin or phycoerythrin/Cy5 were fromSigma and anti-His₆ conjugated to allophycocyanin (APC) from AbCam. Goatanti-mouse conjugated to Percp/Cy5.5 was from Biolegend.

Directed Evolution

The DT40 chicken B cell line AID^(R)CL4 (23) was grown in RPMI-1640 with7% foetal bovine serum and 3% chicken serum at 37° C. and 5% CO2.Transfections were performed by electroporation in 0.4 cm cuvettes usinga Gene Pulser (BioRad) at 250V and 950 μF and stable transfectantsselected with puromycin. Transfected clones in which the Tie2 constructhad integrated into the rearranged Ig locus were identified by PCR asdescribed previously (23). Expression was confirmed by immunoblottingfor the epitope tag, and Tie2 ectodomain and surface expressionconfirmed by immunostaining of non-permeabilized cells.

For ligand binding and fluorescence activated cell sorting DT40 cellswere washed in phosphate buffered saline containing 10% foetal bovineserum and incubated with the appropriate ligands for 30 min at roomtemperature before washing and staining with anti-Ang1, anti-FLAG,anti-His₆ or fluorescently-labelled streptavidin (for biotinylated Ang2detection) and fluorescently-labelled secondary antibodies, asappropriate, at 4° C. Routinely between 50-100 million cells were sortedby FACS and selected cells recovered directly into culture medium forfurther growth. Cells were grown and sorted repeatedly as described inthe Results and Discussion.

In order to sequence the Tie2 surface expression construct exogenouslyexpressed in the DT40 cells genomic DNA was prepared from DT40 cells.The Tie2 ectodomain insert amplified by PCR, cloned into a bacterialsequencing plasmid and transformed into E. coli. Colonies were picked atrandom and plasmids sequenced.

Expression of Soluble Ectodomains

For expression in Hek293 cells, cDNA encoding wild-type Tie2 ectodomain(1-442) was subcloned into pcDNA 3.1 upstream of a human Fc tag andC-terminal His₆ sequence (kindly supplied by Dr Richard Kammerer). Sitedirected mutagenesis was used to modify this wild-type sequence tocorrespond to the evolved mutants. Site directed mutagenesis wasperformed essentially using the QuickChange protocol (AgilentTechnologies) and confirmed by sequencing.

Soluble ectodomain-Fc fusion proteins were obtained by transfection ofHEK293 cells in suspension using polyethylenimine (28) and cells grownfor 3-4 days to allow the fusion proteins to accumulate in the medium.Debris was removed from medium by centrifugation and fusion proteinpurified by Ni-NTA chromatography (Qiagen) followed by buffer exchangeinto tris buffered saline containing 10% glycerol. Proteinconcentrations were determined by Bradford assay. Proteins were storedat 4° C.

Binding Assays

Surface plasmon resonance was performed using a ForteBio Octetinstrument (Pall Life Sciences). Fusion proteins were immobilised at 5μg/ml on sensors and kinetic binding assays perfoinied as detailed bythe manufacturer.

ELISA assays were performed in 96 well plates in which 5 μg/ml Ang1 orAng2 was immobilized. Following blocking with TBS containing 1 mg/ml BSAand 0.1% Triton-X100 different concentrations of fusion protein wereallowed to bind for 1 hour and after washing bound fusion proteinsdetected with anti-Tie2 ectodomain antibodies followed byperoxidase-conjugated secondary antibody and colourimetricquantification.

Cellular Assays

The endothelial cell line EA.hy926 was cultured in DMEM containing 10%foetal bovine serum at 37° C. and 5% CO2. Cells were quiesced byincubation in serum-free medium before activation with Ang1, Ang2 orboth in the absence or presence of 25 μg/ml wild-type or evolvedectodomain-Fc for 30 mins. After washing, cells were lysed and equalamounts of cellular proteins were resolved by SDS/PAGE before detectionof S473-phospho-Akt and total Akt by immunoblotting.

Migration assays were performed in Transwell tissue culture wellscontaining 8 μm pore size inserts (Becton-Dickinson, UK). Serum-freemedium containing 250 μg/ml BSA together with Ang1 or Ang2 in theabsence or presence of soluble ectodomain-Fc fusion protein was placedin the lower chamber of the wells. 10⁵ endothelial cells in serum-freemedium containing 250 μg/ml BSA were placed in the upper chambers andcells were allowed to migrate for 4 h at 37° C. Cells on the uppersurface were gently removed with a cotton bud and the membrane fixed in4% formaldehyde. Membranes were washed in PBS and nuclei stained withDAPI (0.1 μg/ml). Membranes were mounted in glycerol and the numbers ofcells migrating through the membrane were counted magnification in 5random fields on the underside of each insert membrane.

Results and Discussion

Combining cell surface display with the ability of certain B cell linesto diversify genes targeted to immunoglobulin loci could provide apowerful strategy for directed evolution of protein binding and otherfunctions (FIG. 1A). Therefore we used this approach to seek to evolve aTie2 ectodomain that preferentially binds Ang2 better than theprotective ligand Ang1, with which it shares more than 70% amino acidsequence identity in its receptor binding domain (FIG. 1B). To do this acDNA sequence encoding residues 1-442 of the Tie2 ectodomain togetherwith a linker sequence, epitope tag and PDGF receptor transmembranedomain was constructed for surface expression of the ectodomain inB-cells (FIG. 1C). The epitope tag was incorporated to allowquantification of surface expression levels. Previous work has shownthat angiopoietin binding only requires residues 23-210 of Tie2 (24).However, it is known that in other proteins mutations at sites remotefrom the interaction domain can often affect binding ability (25) andfor this reason we included additional portions of Tie2 ectodomainbeyond residue 210 in our directed evolution strategy. The cDNAconstruct was cloned into a vector, pHypermut2 (23), for targetedintegration into the Ig locus of the chicken cell line DT40 (FIG. 6).Chicken B cells normally diversify their Ig loci by a combination ofgene conversion, using an array of upstream IgV pseudogene segments, andby untemplated somatic hypermutation. We used a variant of DT40 in whichthe IgV pseudogene donors have been deleted and that thereforediversifies only by hypermutation (23). Stably transfected clones wereselected for expression of the construct from the rearranged IgV locusby immunoblotting and by PCR (FIG. 7, 8). Surface expression wasverified by ant-FLAG immunofluorescence (FIG. 1D). We also confirmedthat the ectodomain was competent to bind Ang1 and Ang2 by flowcytometry (FIG. 1E). Apparent binding affinities for Ang1 and Ang2 onthe cell surface were derived by incubating with differentconcentrations of Ang1 or Ang2 and flow cytometry (data not shown),revealing an apparent K_(d) for Ang1 of 0.70+/−0.36 nM (n=5) and forAng2 of 2.00+/−0.30 nM (n=3).

In order to evolve Tie2 to preferentially bind Ang2 we used a two-stagestrategy, first aiming to decrease the ability of the ectodomain to bindAng1 and then to test, and if necessary increase, Ang2 binding whilstmaintaining low Ang1 binding. For the first stage cells were incubatedwith Ang1, binding of which was detected by anti-Ang1 andphycoerythrin/Cy5-conjugated secondary antibody, while expression ofTie2 ectodomain construct was monitored with FITC-conjugated anti-FLAG(FIG. 2A). Two subpopulations of cells were observed, one negative forexpression of the ectodomain and binding of Ang1, and the other positivefor both (FIG. 2A). Sequencing of the Tie2 ectodomain construct from thedouble negative population confirmed the cells did retain the construct,but that it contained mutations or deletions that would inactivateexpression (FIG. 9). Cells positive for expression and with the lowestAng1 binding were selected by FACS and expanded (FIG. 2A). After fouriterations of selection and expansion a population of cells withdecreased Ang1 binding compared with parental cells was obtained. Wethen changed the selection strategy to ensure robust Ang2 binding. Weincubated the round 4 cells with Ang1 together and biotinylated Ang2 andmonitored binding of the two ligands with fluorescent secondary reagents(FIG. 2A, lower plots). Cells with highest Ang2 binding and low Ang1binding were selected by FACS. After four rounds of this selection andexpansion regime a population of cells with apparent preferentialbinding to Ang2 was evolved, which we designated R3.

Direct comparison of parental and R3 cells for their ability to bindAng1 and Ang2 was performed for each of the ligands (FIG. 2B). Cells inthe R3 population appeared only able to bind Ang2 and had negligibleAng1 binding whereas parental cells expressing wild-type Tie2 were ableto bind both ligands.

We next obtained sequences encoding the ectodomain that was expressed onthe cells with preferential Ang2 binding (FIG. 2A). Ten sequences weredetermined and all had a common set of changes, specifically F161 wasreplaced by I and there was a tandem deletion of R167 and H168 (FIG.3A). The F/I substitution was the result of a single nucleotide changein the F165 codon from TTC to ATC. The RH double deletion resulted fromloss of the final C of codon P166 together with the CGG encoding R167and the first two nucleotides, CA, of codon H168. This created a newcodon for P166, CCT, and removal of R167/H168 (FIG. 10). In addition tothese changes a number of other mutations were found in the R3population, specifically V154L, P171A, E169D, V170I and T226S (FIG. 10),however none of these were present in all sequences. Interestingly,examination of the published structure of Tie2 ectodomain revealed thatboth the F161I substitution and double R16711168 deletion occur at thebinding interface of the receptor for angiopoietins (FIG. 3B).

In order to analyse the binding characteristics of the evolvedectodomain in more detail we constructed the wild-type ectodomain(residues 1-442) with a carboxy-terminal Fe-tag and introduced the F161Iand ΔR167, 11168 into this sequence by site directed mutagenesis.Wild-type and R3 ectodomains were expressed in HEK293 cells as secretedsoluble proteins of approximately 80 kDa and purified (FIG. 11). Bindingof Ang1 and Ang2 was examined using surface plasmon resonance. Asexpected the wild-type ectodomain bound both ligands (FIG. 4A). Incontrast, the evolved ectodomain was only able to bind Ang2 and showednegligible Ang1 binding (FIG. 4A), consistent with our observation onthis variant ectodomain when expressed on the cell surface (FIG. 2B).The affinity of interaction between Ang2 and the evolved ectodomain(K_(d)=2.4+/−0.3 nM) was similar to that with wild-type ectodomain(4.1+/−0.8 nM (n=3)). Maximal Ang2 binding was lower(B_(max)=0.25+/−0.02 arbitrary units) than with wild-type(B_(max)=0.67+/−0.06 arbitrary units (n=3)). It was surprising to usthat changes at only three residues caused such a dramatic switch in thebinding specificity of the receptor ectodomain. Loss of two residues andthe conservative substitution of I for F are very unlikely to change theimmunogenicity of this protein suggesting the evolved form will be welltolerated by the immune system if used therapeutically.

We were interested to examine the individual effects of the F161Isubstitution and double ΔR167,H168 deletion on binding. We thereforeconstructed wild-type Fc soluble ectodomain with F161I substitution orΔR167,H168 changes and tested Ang1 and Ang2 binding in ELISA assays.There was no distinguishable difference between wild-type andF161I-ectodomains in binding to Ang1 (FIG. 4C). Similarly, Wild-type andF161I-ectodomains both bound equally well to Ang2 (FIG. 4C). Incontrast, deletion of R167/H168 completely abolished ectodomain bindingto Ang1 and Ang2 (FIG. 4C). This was an unexpected result and showedthat the changes introduced by the directed evolution act in a uniquelycombinatorial rather than an additive way to switch the bindingspecificity of the ectodomain.

The crystal structure of Tie2 ectodomain bound to Ang2 shows that F161of Tie2 stacks with F469 in Ang2 (26). The equivalent position in Ang1has a G residue rather than F. Substitution of I for F161 retains thehydrophobic character of this position but would negatively affect thearomatic stacking between F161 in Tie2 and F469 in Ang2. R167 in Tie2appears to make a salt bridge with D448 in Ang2. As D448, andsurrounding sequence, is conserved between Ang2 and Ang1 it would beanticipated that loss of R167 would affect binding to both ligandssimilarly. In the ectodomain H168 forms hydrogen bonds in Ang2 with S417and Y476 and also interacts with P452 (26). In Ang1 S417 is an I whereasY476 and P452 are conserved. However, loss of both R167 and H168 mightbe expected to significantly disrupt or alter the nature of theinterface between Ang1/2 and Tie2.

To test the effects of R3 ectodomain on the cellular actions of Ang2 weexamined its ability to interfere with Ang2 antagonism of Ang1 in theendothelial cell line EA.hy926. Endothelial cells challenged with Ang1showed an activation of Akt and, consistent with the reported antagonisteffects of Ang2, this was suppressed by Ang2 (FIG. 5A). Inclusion ofwild-type ectodomain with Ang1 plus Ang2 further suppressed Aktactivation, as expected from the ability of this fusion protein to bindAng1 as well as Ang2. However, when the evolved ectodomain was addedwith Ang1 and Ang2 the inhibitory effect of Ang2 was reversed and Aktactivation restored (FIG. 5A). To further examine the effects of evolvedectodomain we tested its effects on the agonist activity of Ang2. Highconcentrations of Ang2 have previously been reported to stimulatesignalling in endothelial cells (27) and as shown in FIG. 5B, 1 μg/mlAng2 caused a low level stimulation of Akt phosphorylation in EA.hy926cells. However, whereas the stimulatory effect of Ang1 was unaffected byinclusion of the evolved ectodomain, the agonist activity of Ang2 wasblocked (FIG. 5B). As an additional test of the ability of evolvedectodomain to inhibit Ang2 we examined the agonist activity of high Ang2concentrations on migration of endothelial cells. Both Ang1 at 50 ng/mland Ang2 at 1 μg/ml stimulated endothelial cell migration (FIG. 5C).Addition of the evolved ectodomain did not affect Ang1-induced migrationbut blocked migration in response to Ang2 (FIG. 5C).

Combining surface display with SHM-driven diversification has allowed usto evolve a new form of Tie2 ectodomain with dramatically shiftedbinding specificity and ability to sequester Ang2, a ligand associatedwith a range of pathologies. Soluble forms of this ectodomain bind Ang2preventing it from antagonising Ang1, as well as inhibiting the agonistactivity associated with high concentrations of Ang2. In contrast toAng2, Ang1 has important roles in vascular protection so a molecule thatcan block Ang2 without interfering with Ang1 has significant benefitsfor therapeutic use, particularly in conditions associated withinflammation.

REFERENCES

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Example 2 In Vivo Activity of the Ang2 Ligand Trap R3

The angiopoietins have key roles in regulating vascular inflammation andpermeability (1, 2). Elevated Ang2 has been implicated in inflammationand oedema associated with a range of conditions including sepsis, adultrespiratory distress syndrome and renal failure with multiorgandysfunction (3-5). Ang2 stimulates local inflammatory responsescharacterized by vascular leakage (6) and is an essential mediator ofvascular inflammation and oedema induced by pro-inflammatory cytokinesand other stimuli, including lipopolysaccharide (LPS) (7, 8). To testthe activity of the evolved ectodomain in vivo, therefore, we examinedthe ability of the protein to inhibit localized oedema formation inducedby LPS in mice Animals were injected subcutaneously in the hock withcontrol vehicle, LPS, LPS together with evolved ectodomain (R3) or LPSwith the non-binding ΔR167,H168 ectodomain. As shown in FIG. 13, twohours post-injection LPS produced subcutaneous oedema as measured bysubcutis thickness. However, when administered together with the evolvedectodomain this effect was blocked, consistent with the ability of theectodomain to bind and block Ang2-mediated vascular permeability. Incontrast, the ΔR167,H168 non-binding ectodomain failed to inhibitLPS-induced oedema. A similar experiment was performed to examinelocalized oedema one hour after LPS injection into hocks. Again, theevolved ectodomain (R3) blocked the ability of LPS to induce localizedoedema associated with inflammation (FIG. 14).

Methods

Littermate C57Bl/6 mice (age and sex matched) were taken from coloniesbred in a specific pathogen barrier unit at University of Leicester.Mice were humanely restrained, and received 5 μg LPS (E. coli 0111:B4,TLR grade; Enzo Life Sciences, Inc) with or without 15 μg purifiedevolved ectodomain or control ectodomain protein (in 10 μl volumesdiluted in PBS). The injection site was the mouse hock. The procedurewas compliant with Home Office regulations and institutional guidelines.At different times after injection mice were culled by cervicaldislocation and hocks were prepared for histological analysis (fixation,decalcification using 6% (v/v) trichloroacetic acid in neutral bufferedsaline, and paraffin embedding). 5 μm sections were stained withWright's stain and those selected in which the distance of the tibiaperiost to epidermis could be comparatively measured (using Delta PixInSight (v.3.3.1) imaging software), providing a value of subcutisthickness (local oedema). Nine to 13 data points were obtained from eachsection, blinded for treatment. Statistical analysis was performed byunpaired ‘t’ test of paired data sets, and p<0.05 consideredsignificant.

REFERENCES

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Example 3 Improved Ang2 Ligand-Trap

Using the insight provided by our evolved Tie2 ectodomain (designatedR3) we have generated an additional mutant which may have improved Ang2binding. We generated the new mutant by analysing the possiblemechanisms by which the evolved Ang2-specific ligand trap (deletion ofR167/H168 and substitution of 1161 for F) displays Ang2 specificbinding. Essentially this involved determining possible hydrogenbonding, salt bridges, electrostatic and hydrophobic interactions thatcould contribute to specific Ang2 binding in the evolved ectodomain bycomputationally visualising the published wild-type structure but withthe changes we created in the evolved ectodomain (R3). This led us tohypothesise that a smaller residue at position 161 could furtherincrease Ang2 binding without increasing Ang1 binding. This mutant(deleted at R167 and 11168 and with Glycine at 161) was created,expressed and assayed for binding by SPR (FIG. 15). The new mutantshowed specific binding to Ang2 over Ang1 (as did the evolved R3protein), and a higher level of binding to Ang2 than the originalevolved ectodomain displayed. This new mutant is a development of ouroriginal R3 Ang2-specific ligand trap. The binding to Ang2 was alsoanalysed by ELISA, again showing increased Ang2 binding (FIG. 16).

All publications mentioned in the above specification are hereinincorporated by reference. Various modifications and variations of thedescribed methods and system of the present invention will be apparentto those skilled in the art without departing from the scope and spiritof the present invention. Although the present invention has beendescribed in connection with specific preferred embodiments, it shouldbe understood that the invention as claimed should not be unduly limitedto such specific embodiments. Indeed, various modifications of thedescribed modes for carrying out the invention which are obvious tothose skilled in biochemistry and biotechnology or related fields areintended to be within the scope of the following claims.

The invention claimed is:
 1. A polypeptide comprising a modifiedangiopoietin receptor or fragment thereof, wherein the fragmentcomprises a sequence corresponding to residues 23-210 of SEQ ID NO: 1,wherein the polypeptide binds preferentially to angiopoietin-2 (Ang2)compared to angiopoeitin-1 (Ang1); wherein the angiopoietin receptor isTie2, and wherein the polypeptide comprises the following mutations withrespect to SEQ ID NO: 1 or SEQ ID NO: 2: (i) F161I, ΔR167 and ΔH168, or(ii) F161G, ΔR167 and ΔH168.
 2. A polypeptide according to claim 1,wherein the polypeptide binds to Ang2 and Ang1 with an affinity ratio ofat least 10:1.
 3. A polypeptide according to claim 1, wherein thepolypeptide binds to Ang2 with a K_(d) of less than 10 nM and/or thepolypeptide binds to Ang1 with a K_(d) of greater than 1 μM.
 4. Acomposition comprising a polypeptide according to claim 1 and apharmaceutically acceptable carrier, diluent or excipient.
 5. Apolypeptide according to claim 1, wherein the polypeptide comprises theamino acid sequence of SEQ ID NO:
 3. 6. A polypeptide that bindspreferentially to angiopoietin-2 compared to angiopoietin-1, saidpolypeptide comprising the amino acid sequence of SEQ ID NO: 3.